Modeling of Current-voltage Characteristics of thin film Solar cells

Abstract

Energy crisis and environmental issue lead us to investigate renewable and green energy sources. Abundant source of green energy is sun light, which can be harvested in many different procedures. Solar panel is proved the most effective method for extracting energy from sun light. The second-generation thin film solar cells are increasingly promising for their cheaper production and better efficiency. Cells based on (i) polycrystalline CdTe, (ii) polycrystalline CuInGaSe2 (CIGS) and (iii) hydrogenated amorphous Si (a-Si:H) absorbers are the three most potential photoconductors for thin film solar cells because of their excellent efficiency.
A realistic first principle analytical (preferably) model is essential for properly understanding the operating principles of a solar cell and optimizing its overall efficiency. Until now, few models were proposed in the literature but those models are either oversimplified or having too many fitting parameters. In this thesis we have proposed an analytical model to study the current-voltage characteristics of pin/nip structured cell especially CdS/CdTe thin film solar cells by incorporating exponential photon absorption, carrier trapping and carrier drift in the CdTe layer. An analytical expression for the external voltage dependent photocurrent is derived by solving the continuity equation for both electrons and holes. The overall load current is calculated considering the actual solar spectrum. The recombination current in the depletion region dominates over the ideal diode current in CdTe solar cells. The solar cell efficiency depends critically on the transport properties of the carriers that drift towards the back contact. The photon absorption capability over a wide spectrum and good carrier transport properties of the absorber layer are equally important for achieving higher efficiency. The analytical model shows a very good agreement with the published experimental data on various thin film solar cells. The fitting of the model with the published experimental data considering the actual solar spectrum determines the carrier transport properties (mobility-lifetimes), the amount of reflection and scattering losses in various solar cells.